1. Field of the Invention
This invention relates to an image forming method for forming and developing an electrostatic latent image in electrophotography or electrostatic printing. More particularly, this invention relates to an image forming method which comprises charging by the use of a contact charging means, forming an electrostatic latent image and developing the electrostatic latent image, in which injection charging performance and developing performance can be made stable over a long period of time.
2. Related Background Art
A number of methods are conventionally known as electrophotography. Copies are commonly obtained by forming an electrostatic latent image on a photosensitive member by a charging means and an image exposure means while utilizing a photoconductive material, subsequently developing the latent image with a toner to form a visible image (a toner image), transferring the toner image to a transfer-receiving medium such as paper, and thereafter fixing the toner image to the transfer-receiving medium by heat and/or pressure. Here, the toner not transferred to the transfer medium and remaining on the photosensitive member is removed through a cleaning step from the surface of the photosensitive member.
In recent years, various organic photoconductive materials have been brought out as photoconductive materials for electrophotographic photosensitive members. In particular, function-separated type ones in which a charge generation layer and a charge transport layer are laminated, have been put into practical use, and are mounted on copying machines, printers and facsimile machines. As charging means used in such electrophotographic apparatus, corona discharging has been utilized. Since, however, it causes ozone in a large quantity, the appratus must have a filter, and there have been such problems that the apparatus must be made large in size and the cost increases.
As techniques for solving such problems, charging methods have been proposed in which a charging member such as a roller or a blade is brought into contact with the surface of a photosensitive member so as to form a narrow space in the vicinity of the contact portion, and the discharge as can be explained by what is called the Paschen's law is formed so that the generation of ozone can be prevented as much as possible. In particular, a roller charging system making use of a charging roller as the charging member is preferably used in view of the stability of charging.
This charging is performed by causing discharge from the charging member to the member to be charged, and hence the charging takes place upon application of a voltage above a certain threshold voltage. For example, when a charging roller is brought into contact with a photosensitive member having an about 25 .mu.m thick photosensitive layer and containing an organic photoconductive material, the surface potential of the photosensitive member begins to rise upon application of a voltage of about 640 V or above and, at voltages above that voltage, the photosensitive member surface potential linearly increases at a gradient 1 with respect to the applied voltage. This threshold voltage is hereinafter defined as charging start voltage Vth. Namely, in order to attain a photosensitive member surface potential Vd, an excessive DC voltage of Vd+Vth must be applied to the charging roller. In addition, since any environmental variations may change the resistivity of the charging roller, it has been difficult to control the potential of the photosensitive member to the desired value.
Accordingly, in order to uniform variable charging, as disclosed in Japanese Patent Application Laid-open No. 63-149669, a DC+AC charging system is employed in which a voltage formed by superimposing an AC voltage with a 2.times.Vth or higher peak-to-peak voltage on a DC voltage corresponding to the desired Vd. This is a system aiming at a potential-leveling effect which is attributable to AC, where the potential of the member to be charged converges at the Vd, the middle of the peak of AC voltage, and is hardly affected by external disturbance such as environmental variations.
Even in such contact charging methods, however, their fundamental charging mechanisms utilize the phenomenon of discharge from the charging member to the photosensitive member, and hence the voltage necessary for charging as stated above must be at a value over the surface potential of the photosensitive member. Moreover, the electric field of AC voltage may remarkably cause vibration and noise (hereinafter "AC charging noise") of the charging member and photosensitive member, and the discharge may remarkably cause deterioration of the surface of the photosensitive member. This involves another problem.
As disclosed in Japanese Patent Application Laid-open No. 61-57958, a charging method is known in which a photosensitive member having a conductive protective film is charged using conductive fine particles. This publication discloses that a photosensitive member having a semiconductive protective film having a resistance of from 10.sup.7 to 10.sup.13 .OMEGA..multidot.cm is used and this photosensitive member is charged using conductive fine particles having a resistance of 10.sup.10 .OMEGA..multidot.cm or below whereby the photosensitive member can be evenly and uniformly charged without injection of charges into the photosensitive layer, enabling good images to be reproduced. According to this method, the vibration and noise which has been a problem in AC charging can be prevented, but no sufficient charging efficiency can be achieved, and besides, since, e.g., the conductive fine particles serving as a charging member scrape off the transfer residual toner, the toner may adhere to the charging member and consequently a change in charging performance may occur as a result of many-sheet running.
As a charging method having a good charging efficiency, what is called injection charging is known, in which charges are directly injected into a photosensitive member.
This method, in which a voltage is applied to a contact charging member such as a charging roller, a charging fiber brush or a charging magnetic brush to inject charges into a trap level present on the photosensitive member surface, is disclosed in, e.g., Japan Hardcopy '92 Papers, p. 287, "Performance of Contact Charging Using Conductive Roller". In this method, charges are injected into a dark-portion insulating photosensitive member by means of a low-resistance charging member to which a voltage has been applied. This method has been conditioned on a sufficiently low resistance of the charging member and also on its surface to which a material providing the charging member with a conductivity is sufficiently laid bare. Hence, it is reported also in the above publication that aluminum foil or an ion-conductive charging member made to have a sufficiently low resistivity in a high humidity environment is preferable as the charging member. Studies made by the present inventors have revealed that the resistivity of charging members at which charges can be sufficiently injected into photosensitive members is 1.times.10.sup.3 .OMEGA..multidot.cm or below and, at a resistivity higher than that, a difference begins to occur between applied voltage and charge to cause a problem on the convergence of charge potential.
However, when the charging member having such a low resistivity is actually used, excess leak currents may flow from the charging member to scratches and pinholes produced on the photosensitive member surface to tend to cause faulty charging around them, expansion of the pinholes and electrification failure of the charging member.
To prevent such difficulties, it is necessary to make the charging member have a resistivity of about 1.times.10.sup.4 .OMEGA..multidot.cm or above. However, as stated previously, the charging member having this resistivity leads to such an inconsistency that the performance of charge injection into the photosensitive member may lower and sufficient charging cannot be effected.
Accordingly, with regard to contact type charging assemblies or image forming methods making use of such charging assemblies, it has been sought to solve the above problems, i.e., to simultaneously achieve conflicting performances of preventing the photosensitive member surface from pinhole leak which has not been able to be prevented in low-resistivity charging members and of performing sufficient charge injection.
As stated above, in the image forming method having the charging step making use of the charging member brought into contact with the photosensitive member, any faulty charging due to contamination of the charging member tends to cause faulty images and problems in running durability may arise. Thus, also in the charging carried out by injecting charges into the photosensitive member, it has been a pressing need for the realization of many-sheet printing to prevent the influence of the faulty charging due to contamination of the charging member.
Thus, the present inventors, as a result of extensive studies of the surface layers of latent image bearing members such as photosensitive members used when charged by charge injection and of contact charging members, have discovered that a sufficient charging performance can be attained when the contact charging member is made to have a volume resistivity (B) of from 10.sup.4 to 10.sup.9 .OMEGA..multidot.cm and the surface layer of a latent image bearing member a volume resistivity (A) of from 10.sup.8 to 10.sup.15 .OMEGA..multidot.cm, and such members are preferred.
However, it has been understood that the charge injection performance lowers and no satisfactory images can be formed when materials having a volume resistivity of 10.sup.14 .OMEGA..multidot.cm or above such as silica used as what is called toner external additives are once taken into the contact charging member. This problem is serious especially in the case of cleanerless systems, in which no cleaning means for removing transfer residual toner is provided between the transfer zone and the charging zone.
It has also been understood that no satisfactory images can be formed because, when the surface layer of a latent image bearing member has a volume resistivity (B) of about 1.times.10.sup.8 .OMEGA..multidot.cm as stated above, electrostatic latent images formed on the latent image bearing member are disordered in the developing zone by rubbing friction with a two component type developer making use of an iron powder carrier or a commonly known ferrite carrier such as copper-zinc ferrite or nickel-zinc ferrite having a volume resistivity of from 1.times.10.sup.8 to 1.times.10.sup.10 .OMEGA..multidot.cm.